> The more you tax flying, the more you make it only a rich person's activity.
Why is that a problem? Climbing the Mount Everest, Joining a Country Club, or flying into space is also a rich person's activity - is that a problem too?
> Why only international flights
AFAIK some countries already tax domestic flights. And if they don't, that's easy to achieve. Taxing international flights is harder, because there'll always be a few countries that won't agree.
> However, very difficult to achieve politically.
Yes. But "difficult to achieve politically" is strange: because the only hurdle to take is not a 'real' hurdle. Not 'real' as most engineering, or economic hurdles are. It's not a matter of money, funds, scientific breakthroughs or such, it's mere a matter of willingness to change. Which, it appears, is one of the hardest hurdles to take.
And yes: I agree we should not leave any percent improvement on the table. But I remain with my point that money and effort that we can spend only once, should not go to changing the habits, cultutre and lifestyle of 1/4th of the worlds' population for a meagre 2% win, but to changing the habits, culture and lifestyle of another part of the population can get us a 15% win.
Ideally we'd be able to spend that effort and money twice. For a total 17% win. But as long as we cannot do that, let's spend it on where it gets the biggest results.